Roll your own IRC bot

This tutorial is designed as a practical guide to writing real world code in Haskell and hopes to intuitively motivate and introduce some of the advanced features of Haskell to the novice programmer. Our goal is to write a concise, robust and elegant IRC bot in Haskell.

Getting started

You’ll need a reasonably recent version of GHC or Hugs. Our first step is to get on the network. So let’s start by importing the Network package, and the standard IO library and defining a server to connect to.

The key here is the main function. This is the entry point to a Haskell program. We first connect to the server, then set the buffering on the socket off. Once we’ve got a socket, we can then just read and print any data we receive.

Put this code in the module 1.hs and we can then run it. Use whichever system you like:

Now, we’ve done quite a few things here. Firstly, we import Text.Printf, which will be useful. We also set up a channel name and bot nickname. The main function has been extended to send messages back to the IRC server using a write function. Let’s look at that a bit more closely:

We’ve given write an explicit type to help document it, and we’ll use explicit types signatures from now on, as they’re just good practice (though of course not required, as Haskell uses type inference to work out the types anyway).

The write function takes 3 arguments; a handle (our socket), and then two strings representing an IRC protocol action, and any arguments it takes. write then uses hPrintf to build an IRC message and write it over the wire to the server. For debugging purposes we also print to standard output the message we send.

This function takes a Handle argument, and sits in an infinite loop reading lines of text from the network and printing them. We take advantage of two powerful features; lazy evaluation and higher order functions to roll our own loop control structure, forever, as a normal function! forever takes a chunk of code as an argument, evaluates it and recurses – an infinite loop function. It is very common to roll our own control structures in Haskell this way, using higher order functions. No need to add new syntax to the language, lisp-like macros or meta programming – you just write a normal function to implement whatever control flow you wish. We can also avoid do-notation, and directly write: forever a = a >> forever a.

We add 3 features to the bot here by modifying listen. Firstly, it responds to PING messages: if ping s then pong s .... This is useful for servers that require pings to keep clients connected. Before we can process a command, remember the IRC protocol generates input lines of the form:

so we need a clean function to simply drop the leading ‘:’ character, and then everything up to the next ‘:’, leaving just the actual command content. We then pass this cleaned up string to eval, which then dispatches bot commands.

So, if the single string “!quit” is received, we inform the server and exit the program. If a string beginning with “!id” appears, we echo any argument string back to the server (id is the Haskell identity function, which just returns its argument). Finally, if no other matches occur, we do nothing.

We add the privmsg function – a useful wrapper over write for sending PRIVMSG lines to the server.

Roll your own monad

A small annoyance so far has been that we’ve had to thread around our socket to every function that needs to talk to the network. The socket is essentially immutable state, that could be treated as a global read only value in other languages. In Haskell, we can implement such a structure using a state monad. Monads are a very powerful abstraction, and we’ll only touch on them here. The interested reader is referred to All About Monads. We’ll be using a custom monad specifically to implement a read-only global state for our bot.

The key requirement is that we wish to be able to perform IO actions, as well as thread a small state value transparently through the program. As this is Haskell, we can take the extra step of partitioning our stateful code from all other program code, using a new type.

Firstly, we define a data type for the global state. In this case, it is the Bot type, a simple struct storing our network socket. We then layer this data type over our existing IO code, with a monad transformer. This isn’t as scary as it sounds and the effect is that we can just treat the socket as a global read-only value anywhere we need it. We’ll call this new io + state structure the Net monad. ReaderT is a type constructor, essentially a type function, that takes 2 types as arguments, building a result type: the Net monad type.

We can now throw out all that socket threading and just grab the socket when we need it. The key steps are connecting to the server, followed by the initialisation of our new state monad and then to run the main bot loop with that state. We add a small function, which takes the intial bot state and evaluates the bot’s run loop “in” the Net monad, using the Reader monad’s runReaderT function:

loop st = runReaderT run st

where run is a small function to register the bot’s nick, join a channel, and start listening for commands.

While we’re here, we can tidy up the main function a little by using Control.Exception.bracket to explicitly delimit the connection, shutdown and main loop phases of the program – a useful technique. We can also make the code a bit more robust by wrapping the main loop in an exception handler using catch:

That is, the higher order function bracket takes 3 arguments: a function to connect to the server, a function to disconnect and a main loop to run in between. We can use bracket whenever we wish to run some code before and after a particular action – like forever, this is another control structure implemented as a normal Haskell function.

Rather than threading the socket around, we can now simply ask for it when needed. Note that the type of write changes – it is in the Net monad, which tells us that the bot must already by connected to a server (and thus it is ok to use the socket, as it is initialised).

So we now have a bot with explicit read-only monadic state, error handling, and some basic IRC operations. If we wished to add read-write state, we need only change the ReaderT transformer to StateT.

Extending the bot

Let’s implement a basic new command: uptime tracking. Conceptually, we need to remember the time the bot starts. Then, if a user requests, we work out the total running time and print it as a string. A nice way to do this is to extend the bot’s state with a start time field:

import System.Time

data Bot = Bot { socket :: Handle, starttime :: ClockTime }

We can then modify the initial connect function to also set the start time.

That is, in the Net monad, find the current time and the start time, and then calculate the difference, returning that number as a string. Rather than use the normal representation for dates, we’ll write our own custom formatter for dates:

Where to now?

This is just a flavour of application programming in Haskell, and only hints at the power of Haskell’s lazy evaluation, static typing, monadic effects and higher order functions. There is much, much more to be said on these topics. Some places to start: